1. Field of the Invention
The present invention pertains to an apparatus and method for diagnosing a breathing disorder, and, in particular, to an apparatus that determines a dimension of a portion of a patient""s airway, such as a cross-sectional area of the airway, for each of a plurality of internal loads imposed on the patient""s airway and, from this information, assesses the likelihood that the patient suffers from a breathing disorder. The present invention also pertains to a method of diagnosing the likelihood that a patient suffers from a breathing disorder using such an apparatus.
2. Description of the Related Art
Obstructive sleep apnea (xe2x80x9cOSAxe2x80x9d) is a condition in which sleep is repeatedly interrupted by an inability to breathe due to an obstructed upper airway segment. Those afflicted with OSA experience sleep fragmentation and complete or nearly complete cessation of ventilation intermittently and repeatedly during sleep with potentially severe degrees of oxyhemoglobin desaturation. These symptoms may translate clinically into extreme daytime sleepiness, cardiac arrhythmias, pulmonary-artery hypertension, congestive heart failure and/or cognitive dysfunction. Other consequences of OSA include right ventricular dysfunction, carbon dioxide retention during wakefulness, as well as during sleep, and continuous reduced arterial oxygen tension. Hypersomnolent sleep apnea patients may be at risk for excessive mortality from these factors as well as by an elevated risk for accidents while driving and/or operating potentially dangerous equipment.
Using conventional techniques, OSA cannot be readily diagnosed in a physician""s office or in a hospital during a short visit. Instead, present methods for diagnosing OSA rely on polysomnography to measure the patient""s apnea/hypopnea index (xe2x80x9cAHIxe2x80x9d), which takes place over a period of several hours while the patient is asleep, typically during an overnight sleep monitoring session. This conventional diagnostic technique commonly takes place in a sleep laboratory, which is in a hospital or a clinic. Portable polysomnography devices exist that enable the sleep monitoring session to take place at the patient""s home. However, home monitoring requires that the patient place the monitoring system and sensors on himself or herself and operate the polysomnography monitoring device, which may result in erroneous or inefficient placement of the sensors and/or improper use of the monitor. A caregiver may assist the patient at home in placing the sensors and operating the monitoring system. However, this is costly and time consuming.
It can be appreciated that the conventional overnight procedure for diagnosing OSA, whether in a sleep lab or at home, is time consuming for both the patient and the technician monitoring the patient. This diagnostic technique is also generally inconvenient and may be unsettling to the patient because it typically requires that the patient stay in the hospital or the clinical setting overnight and that the patient wear a myriad of sensors while trying to sleep. The likelihood that the monitoring session will be unsettling is especially true for children and patient""s with an elevated level of apprehension concerning medical facilities, such as patient""s with infirm mental abilities.
Conventional OSA diagnostic procedures also requires a significant amount of processing of complex physiological data by a trained technician, which, again, is time-consuming and costly. Thus, conventional diagnostic procedures, because of their overly burdensome and perhaps onerous techniques, as well as exorbitant expense, are not suitable as a prescreening tool to determine if there is even a likelihood that the patient may suffer from OSA. Rather, conventional diagnostic techniques are best used if it is already determined that there is a likelihood that the patient suffers from OSA. In which case, the conventional techniques can be more appropriately targeted to functions such as determining an appropriate therapy to treat the patient. A practical and reliable prescreening technique would identify patients who are not likely to suffer from OSA so that they can avoid the relatively time consuming and burdensome overnight diagnostic session and also identify those patients who are likely suffers from OSA so that additional diagnosis (if necessary) and treatment can be obtained.
Indications for sleep apnea are generally a patient""s complaint or the complaints of a spouse or a sleep partner of daytime somnolence and/or frequent awakening episodes while sleeping or trying to sleep. These indicators, however, are highly subjective and may be attributable to other factors. In addition, some OSA sufferers, including children and patients afflicted with Down""s Syndrome, have difficulty clearly communicating the nature of their problem with sleep interruption. Thus, these indicators, whether elucidated from the patient via a written questionnaire or orally, are not entirely reliable as a prescreening indication of OSA. Also, physical indicators, such as increased lung residual volume or subsequent cardiovascular complications, are also generally not strong indicators of OSA.
Accordingly, it is an object of the present invention to provide an apparatus for diagnosing, at least for prescreening purposes, whether a patient is likely to suffer from OSA that overcomes the shortcomings of conventional diagnostic devices and techniques. This object is achieved by providing an apparatus for reliably diagnosing the likelihood that the patient suffers from a breathing disorder, such as OSA, during a relatively short period of time and that is performed while the patient is awake, i.e., without the need for a sleep study. The diagnostic device of the present invention includes a first sensing system that determines a dimension of a portion of the patient""s airway, such as the cross-sectional area, and an internal load determination system that determines an internal load imposed on the patient""s airway. An internal load is the force acting on the inside walls of the patient""s airways that tend to urge the airway to collapse, such as a negative pressure within the patient""s airway relative to ambient atmospheric pressure. The internal load can be defined in terms of the negative pressure within the patient""s airway relative to ambient pressure or in terms of the flow of gas within the airway, because there can exist a proportional, known, or determinable relationship between pressure and flow.
The diagnostic device of the present invention also includes a processing system that receives from the first sensing system and the internal load determination systems and determines a compliance curve that defines a relationship between the dimension information determined by the first sensing system and the different internal loads imposed on the patient""s airway detected by the internal load determination system. The compliance curve is indicative of the compliance of the patient""s airway because it represents how the dimensions of the airway change with changes in the internal load imposed on the airway. The processing system also determines at least one characteristic of the compliance curve, such as the slope and/or Y-intercept, indicative of a likelihood that the patient suffers from the breathing disorder.
It is a further object of the present invention to provide a diagnostic device that can be used to assess the likelihood that the patient suffers from a breathing disorder while the patient is seated and awake. In one embodiment, this object is achieved by using a device that applies an external load on the patient""s airway while the patient is sitting, in conjunction with the diagnostic device discussed above. Providing an external load on the patient""s airway enhances the ability of the diagnostic device discussed above to distinguish between OSA sufferers and non-OSA sufferers. Devices suitable to apply an external load on the patient""s airway include: a neck collar with a selectively adjustable circumference, a neck collar with a selectively inflatable bladder that inflates to impose an external force on the patient""s airway, and a neck cuirass that provides a chamber, which is sealed against the patient, so that providing a positive pressure greater than ambient atmospheric within the chamber likewise applies the positive pressure on the front exterior neck tissue of the patient.
In another embodiment of the present invention, assessing the likelihood that the patient suffers from a breathing disorder while the patient is seated and awake is achieved by providing a muscle stimulating device that stimulates the muscles associated with the patient""s airway. The likelihood that the patient suffers from OSA can be determined by observing the patient""s compliance curve in the absence of stimulation and in the presence of stimulation. There is less of a difference between the compliance curves in the presence and absence of muscle stimulation in OSA sufferers than in non-OSA sufferers. In other words, providing muscle stimulation has not significant effect on the compliance curve of an OSA sufferer. For a non-OSA sufferer, however, the muscle stimulation has a relatively large impact on the compliance curve. The more significant the difference between the two compliance curves, i.e., the compliance curve in the absence of muscle stimulation and the compliance curve in the presence of stimulation, the less likely it is that the patient suffers from OSA.
It is yet another object of the present invention to provide a diagnostic device that operates in conjunction with an mechanical device for providing an internal load on the patient""s airway. In one embodiment, this is accomplished by providing a negative pressure generator and cuirass that delivers the negative pressure to the torso of the patient, thereby distending the patient""s chest to create an inspiratory flow. In another embodiment of the present invention, an internal load is provided by applying an negative pressure to an airway of the patient. Using a mechanically generated internal load makes is possible to generate a uniform and easily controlled internal load in the patient""s airway, while minimizing the patient""s level of interaction needed to take the necessary measurements.
It is a still further object of the present invention to provide a method of diagnosing a breathing disorder that overcomes the shortcomings of conventional diagnostic methods. This object is achieved by providing a method for diagnosing a breathing disorder that includes the steps of causing a plurality of internal loads to be imposed on the patient""s airway, determining an amount indicative of each internal load, determining a dimension, such the cross-sectional area, of the patient""s airway at each internal load, determining a compliance curve indicative of the relationship between the dimension of the patient""s airway and the plurality of the internal loads, and determining at least one characteristic associated with the compliance curve, such as the slope and/or Y-intercept, indicative of a likelihood that the patient suffers from a breathing disorder. In a further embodiment of the present invention, a mechanical device is used to generate an internal load on the airway of the patient, by, for example, providing an negative pressure to the airway of the patient or a chest distending force to the patient""s chest area.
These and other objects, features and characteristics of the present invention, as well as the methods of operation and functions of the related elements of structure and the combination of parts and economies of manufacture, will become more apparent upon consideration of the following description and the appended claims with reference to the accompanying drawings, all of which form a part of this specification, wherein like reference numerals designate corresponding parts in the various figures. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only and are not intended as a definition of the limits of the invention.